A device for detecting a positional relationship between opposed first and second objects. The device includes a light source for projecting light to the first and second objects; a photodetecting system for detecting two lights from one of the first and second objects illuminated by the light from the light source, the photodetecting system detecting those two lights the position of incidence of each of which on a predetermined plane is changeable with the positional relationship between the first and second objects in a direction perpendicular to the direction in which the first and second objects are opposed; a position detecting system for detecting the positional relationship between the first and second objects in a direction perpendicular to the opposing direction of the first and second objects, on the basis of the relationship of the two lights, on the predetermined plane, as detected by the photodetecting system; and an interval detecting system for detecting the positional relationship between the first and second objects in the opposing direction, by using at least one of the two lights detected by the photodetecting system.